This invention relates generally to aircraft approach and landing procedures, and more specifically to methods and systems for radar aided aircraft positioning for approaches and landings.
Conventional instrument landing systems (ILS) utilize received localizer and glide slope transmissions to help guide an aircraft to properly approach an aircraft landing area, for example, a runway of an airport. The localizer signal provides azimuth, or lateral, information to be received by the ILS for use in guiding the aircraft to the centerline of the runway. The localizer signal includes radial information for a single course; the runway heading. The glide slope signal is the signal that provides vertical guidance to the aircraft during an ILS approach. A standard glide-slope path is 3° downhill to the approach-end of the runway.
A decision height for an ILS approach is a point on the glide slope where a decision must be made to either continue the landing or execute a missed approach. In part, ILS systems have been categorized as follows: Category I ILS procedures have decision heights of not less than 200 feet and visibility minima not less than 800 m (one-half mile), Category II ILS procedures have decision heights of not less than 100 feet and visibility minima not less than 400 m (one-quarter mile), and one embodiment of Category III ILS procedures have decision heights of not less than 50 feet and visibility minima not less than 200 m (one-eighth mile). Other Category III ILS procedures include a zero decision height. Category III ILS are subcategorized as IIIa, IIIb, and IIIc based on varying visibilities and decision heights. Categorization of ILS systems are based in part on the quality of the radio signals being transmitted.
As can be deduced from the above, the better the quality of the ILS signals, the higher the categorization with the best being Category IIIc and the worst being Category I. For a pilot to be able to make an approach to a runway in bad weather, the weather has to be at or above certain conditions depending on the category of the ILS. If the weather is below the categorization of the ILS, the pilot has to revert to a holding pattern hoping that the weather improves to the capability of the ILS or divert to land elsewhere. No approach can be made to airports where the environments is below the ILS categorization.
Not all runway ends support Category III landings. For a variety of reasons, ILS systems certified to Category III or even Category II supporting certain aircraft approaches are unfeasible or impractical.